1. Field of the Invention
This invention relates to novel diallyl terephthalate prepolymer (hereinafter referred to as "DAT-P") and a process for producing the same by polymerizing diallyl terephthalate (hereinafter referred to as "DAT").
2. Description of the Related Art
Resin made by polymerization of diallyl phthalate (hereinafter referred to as "DAP") as the monomer, which is an isomer of DAT, has been highly rated for its stable electrical properties in a high-temperature, high-moisture atmosphere, and has found wide use in forming materials, laminates, and decorative laminates and the like.
Nevertheless, there is a growing demand for a greater heat-resistance of this resin while preserving the desired properties thereof, to enable a wider use thereof in machines and electrical parts.
In the production of a DAP resin, polymerization is temporarily discontinued before the polymer is substantially gelled, and the polymer thus produced is separated from the system as a prepolymer, which is then further polymerized, after a polymerization initiator or filler is added thereto, into the final product. This is because the monomer has two double bonds in the molecule thereof and thus tends to be gelled, in the ordinary polymerization process, before it is sufficiently polymerized. For such an approach to be economically viable, it is essential to be able to produce the prepolymer in an efficient manner, but at present, the prepolymer production process is not fully developed, and a more efficient approach has been demanded by industry.
Polymerization of DAT as the monomer, which is highly symmetrical, has a potential for the production of heat-resistant polymers but has not been commercialized because the cured product of DAT-P produced by the currently available process is fragile and generally has unsatisfactory mechanical properties.
The prepolymer is usually prepared by bulk polymerization, which uses no solvent, or solution polymerization with the aid of a variety of solvents. Bulk-polymerized prepolymer has a minimum iodine value of about 80, even when polymerization is effected immediately before gellation starts, and the weight-average molecular weight (Mw) thereof determined by gel permeation chromatography (GPC) is very high. The inventors have found that a prepolymer having an excessively high iodine number and Mw, when cured, does not improve the product properties; the major drawbacks of the polymer of hardness and fragility remain, and the viscosity during the forming process is increased to no useful purpose.
Solution polymerization, with the aid of a solvent such as benzene or ethyl acetate, is also known. A solution-polymerized prepolymer has a lower iodine number than a bulk-polymerized prepolymer but still has the above drawbacks, because the number of the monomer units in the carbon-carbon bonded molecular chain portion, which is composed of the allyl group for the monomer units, is still large.
Another approach to the production of a prepolymer which can provide a cured product having a high heat resistance and mechanical strength is a copolymerization of DAT and an aromatic hydrocarbon compound having at least one hydrogen atom on the benzyl position of toluene or xylene, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 59-80409. Such an approach, however, needs special considerations for the process and reactor; e.g., the monomer, polymerization initiator, and aromatic hydrocarbon compound must be injected from specially designed nozzles while being stirred at a high speed. Furthermore, large quantities of polymerization initiator are needed, and recycle of the monomer in a copolarization process is very difficult, although not impossible.
Still another approach is a polymerization of a halogenated hydrocarbon compound or aldehyde derivative having a relatively high chain transfer constant. One of the major problems involved in this approach is the production of radicals from the solvent, which tend to be added to the allyl group to initiate telomerization and degrade the polymer properties.
Diallyl isophthalate resin is known for a high heat-resistance, and is already produced on a commercial scale, but this is impracticable as the monomer is very expensive.
Use of a special monomer, such as naphthalenedicarboxylic acid diallyl ester, has been disclosed, but this monomer is also impracticably expensive.
All of the approaches described above can be improved from the viewpoint of the productivity of the prepolymer.